Add support for armv7-win32-vs11

Add support for armv7-win32-vs11

The arm assembly files are named .s after conversion, to reuse
as much of the existing makefile infrastructure for conversion to
gas format as possible. Within the generated visual studio project,
only the converted assembly sources are available, which might not
be optimal for actually developing it, but is acceptable for
just building the library.

Multithreading is disabled since the traditional win32 threading
functions aren't available on WinRT/Windows Phone 8.

Building of vpx itself and the examples succeed, while building the
tests fail due to them using functions not available in the
windows store/windows phone API subsets - therefore the unit tests
are disabled.

This works for building in Visual Studio Express 2012 for Windows
Phone, while Visual Studio Express 2012 for Windows 8 (for
"Windows Store" apps) seems to reject the vcxproj files due to
not supporting "classic style native application or managed
projects". The built static library should be compatible with that
platform though.

Change-Id: Idcd7eca60bfaaaeb09392a9cd7a02e4a670e3b30